An inquiry into the attack on the US mission, which saw the US ambassador and three other US citizens killed, found that the State Department security arrangements there were "grossly inadequate".

The months-long probe found there had been "no immediate, specific" intelligence of a threat against the mission, which was overrun on September 11 by dozens of heavily armed fighters who killed the four Americans.

She said in a letter to US congressional committees on Tuesday that she had instructed the State Department to implement its findings "quickly and completely" and outlined a series of steps aimed at improving the security of US diplomatic outposts.

The US would send hundreds of additional Marine guards to overseas posts, ask for more money for security improvements and name a new State Department official to oversee "high threat posts," she said.

Domestic grumblings

Reporting from Washington DC, Al Jazeera's Rosiland Jordan said the report puts a dent in Hillary Clinton's record at the State Department.

"This incident obviously happened at a key moment in her tenure," Jordan said.

"There's been much speculation about her future once she steps down," she said about to Clinton's presidential prospects in 2016.

"...there was no protest prior to the attacks, which were unanticipated in their scale and intensity"

- Accountability Review Board report

"There have been a lot of domestic grumblings on how she and the Obama administration responded to the political changes brought about by the Arab awakening."

Citing the report, Jordan said "there wasn't that sense of urgency" at the State Department to secure the diplomatic facilities particularly in countries undergoing political upheavals and revolutions.

The report found that there were "systemic failures and leadership and management deficiencies at senior levels within two bureaus of the State Department resulted in a Special Mission security posture that was inadequate for Benghazi and grossly inadequate to deal with the attack that took place."

The Accountability Review Board (ARB) also concluded "there was no protest prior to the attacks, which were unanticipated in their scale and intensity."

The attacks, in which the consulate and a nearby safe house were targeted, have become fiercely politicised, with Republicans skewering the administration for security failings as well as a possible cover-up over Al-Qaeda's role.